The research described in this article has been funded wholly or in part by the U.S. Environmental Protection Agency, Office of Research and Development, through grants R819305 and R823319 to Portland State University.

Atmospheric free radicals hydroxyl and hydroperoxyl (OH and HO₂, collectively HOᵪ ) are the catalysts that cause secondary or photochemical air pollution. Chemical mechanisms for oxidant and acid formation, on which expensive air pollution control strategies are based, must accurately predict these radical concentrations. We have used the fluorescence assay with gas expansion (FAGE) technique to carry out the first simultaneous, in situ measurements of these two radicals in highly polluted air during the Los Angeles Free Radical Experiment. A complete suite of ancillary measurements was also made, including speciated hydrocarbons, carbon monoxide, aldehydes, nitric oxide, nitrogen dioxide, and ozone along with meteorological parameters. Using this suite of measurements, we tested the ability of a lumped chemical mechanism to accurately predict radical concentrations in polluted air. Comparison of model predictions with measured radical concentrations revealed generally good agreement for OH early and late in the day, including the early evening hours, when OH persisted at low concentrations after dark. During midday, however, modeled [OH] was high by about 50%. Agreement for HO₂ was quite good in the early morning hours, but model-calculated HO₂ concentrations were significantly too high during midday. When we used our measured HO₂ concentrations as model input, agreement between calculated and measured OH concentrations was improved. It seems likely that (1) the model's HOᵪ sources are too large, (2) there are unaccounted HOᵪ loss processes in Los Angeles air, and/or (3) the complex parameterization of RO₂/HO₂ radical chemistry in the reaction mechanism does not adequately describe the behavior of these radicals in the Los Angeles atmosphere.